It is well known to provide a tank vehicle with liquid level detectors in the or each tank of the vehicle so as to warn of overfilling during a filling operation. Such detectors are particularly necessary when the liquid being filled is flammable or otherwise hazardous. The detectors are connected to a monitor unit associated with the filling installation which responds to an alarm condition by cutting off the supply of liquid being filled. A number of such detecting systems are known, and the construction of the detectors themselves forms no part of the present invention. The principal shortcoming of known systems resides in the means used to transmit the alarm signals from the vehicle to the stationary installation. This is normally a flexible cable plugged into a receptacle on the vehicle, and it is found that the plug-in connection is a prolific source of problems whilst the cable is subject to considerable wear and tear. Moreover, plugging in the cable is an additional step in the filling operation. Further problems arise from the use of incompatible connectors in different detection systems, and the special precautions needed when establishing electrical connections in the vicinity of highly flammable liquids.
Conventional methods of wireless communication are unsuitable for establishing the link since filling installations are normally arranged to service several vehicles simultaneously at closely spaced locations and it is essential that the monitoring signals transmitted by each vehicle are reliably received at the correct station but at no other station. This problem is difficult to solve by encoding the signals to identify individual vehicles, since any vehicle may be received to fill at any location and thus each receiver would have to be programmed individually for each vehicle scheduled to use its associated filling location. There are strict limits to the extent that the signals can be beamed, since the transmitters on different vehicles during use may be in widely differing locations relative to the receivers, and even if satisfactory control of the beam characteristics could be achieved at an economic price, scattering and reflection of the signal might still cause reception by the wrong receiver.